CCR3: a key to mast cell phenotypic and functional diversity?

Key mediators in the immunopathogenesis of allergic asthma

Asthma is described as a chronic inflammatory disorder of the conducting airways. It is characterized by reversible airway obstruction, eosinophil and Th2 infiltration, airway hyper-responsiveness and airway remodeling. Our findings to date have largely been dependent on work done using animal models, which have been instrumental in broadening our understanding of the mechanism of the disease. However, using animals to model a uniquely human disease is not without its drawbacks. This review aims to examine some of the key mediators and cells of allergic asthma learned from animal models and shed some light on emerging mediators in the pathogenesis allergic airway inflammation in acute and chronic asthma.

CpG methylation at GATA elements in the regulatory region of CCR3 positively correlates with CCR3 transcription

DNA methylation may regulate gene expression by restricting the access of transcription factors. We have previously demonstrated that GATA-1 regulates the transcription of the CCR3 gene by dynamically interacting with both positively and negatively acting GATA elements of high affinity binding in the proximal promoter region including exon 1. Exon 1 has three CpG sites, two of which are positioned at the negatively acting GATA elements. We hypothesized that the methylation of these two CpGs sites might preclude GATA-1 binding to the negatively acting GATA elements and, as a result, increase the availability of GATA-1 to the positively acting GATA element, thereby contributing to an increase in GATA-1-mediated transcription of the gene. To this end, we determined the methylation of the three CpG sites by bisulfate pyrosequencing in peripheral blood eosinophils, cord blood (CB)-derived eosinophils, PBMCs, and cell lines that vary in CCR3 mRNA expression. Our results demonstrated that methylation of CpG sites at the negatively acting GATA elements severely reduced GATA-1 binding and augmented transcription activity in vitro. In agreement, methylation of these CpG sites positively correlated with CCR3 mRNA expression in the primary cells and cell lines examined. Interestingly, methylation patterns of these three CpG sites in CB-derived eosinophils mostly resembled those in peripheral blood eosinophils. These results suggest that methylation of CpG sites at the GATA elements in the regulatory regions fine-tunes CCR3 transcription.

Eosinophil development, regulation of eosinophil-specific genes, and role of eosinophils in the pathogenesis of asthma

Eosinophils arise from hematopoietic CD34⁺ stem cells in the bone marrow. They acquire IL-5Rα on their surface at a very early stage during eosinophilopoiesis, and differentiate under the strong influence of interleukin (IL)-5. They then exit to the bloodstream, and enter the lung upon exposure to airway inflammatory signals, including eotaxins. In inflamed tissues, eosinophils act as key mediators of terminal effector functions and innate immunity and in linking to adaptive immune responses. Transcription factors GATA-1, CCAAT/enhancer-binding protein, and PU.1 play instructive roles in eosinophil specification from multipotent stem cells through a network of cooperative and antagonistic interactions. Not surprisingly, the interplay of these transcription factors is instrumental in forming the regulatory circuit of expression of eosinophil-specific genes, encoding eosinophil major basic protein and neurotoxin, CC chemokine receptor 3 eotaxin receptor, and IL-5 receptor alpha. Interestingly, a common feature is that the critical cis-acting elements for these transcription factors are clustered in exon 1 and intron 1 of these genes rather than their promoters. Elucidation of the mechanism of eosinophil development and activation may lead to selective elimination of eosinophils in animals and human subjects. Furthermore, availability of a range of genetically modified mice lacking or overproducing eosinophil-specific genes will facilitate evaluation of the roles of eosinophils in the pathogenesis of asthma. This review summarizes eosinophil biology, focusing on development and regulation of eosinophil-specific genes, with a heavy emphasis on the causative link between eosinophils and pathological development of asthma using genetically modified mice as models of asthma.

Kit (W-sh) mice develop earlier and more severe experimental autoimmune encephalomyelitis due to absence of immune suppression

Mast cells (MCs) have been thought to play a pathogenic role in the development of autoimmune diseases, including experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. However, an immunoregulatory function of these cells has recently been suggested. We investigated the role of MCs in EAE using the W(-sh) mouse strain, which is MC deficient. W(-sh) mice developed earlier and more severe clinical and pathological disease with extensive demyelination and inflammation in the CNS. The inflammatory cells were mainly composed of CD4(+) T cells, monocyte/macrophages, neutrophils, and dendritic cells. Compared with wild-type mice, MC-deficient mice exhibited an increased level of MCP-1/CCR2 and CD44 expression on CD4(+) T cells in addition to decreased production of regulatory T cells, IL-4, IL-5, IL-27, and IL-10. We also found that levels of IL-17, IFN-γ, and GM-CSF were significantly increased in peripheral lymphocytes from immunized W(-sh) mice compared with those in peripheral lymphocytes from wild-type mice. Reconstitution of W(-sh) mice downregulated susceptibility to EAE, which correlated with MC recruitment and regulatory T cell activation in the CNS. These findings indicate that responsiveness is not required in the pathogenesis of inflammatory demyelination in the CNS and that, in the absence of MCs, increased MCP-1, CCR2, IL-17, IFN-γ, CD44, and other inflammatory molecules may be responsible for increased severity of EAE.

Beta‐chemokine receptors 5 and 3 are expressed on the head region of human spermatozoon

Induction of human sperm chemotaxis is an established phenomenon, though signaling systems physiologically involved have not been identified. Recently, it has been demonstrated that RANTES is present in the follicular fluid and that this molecule is a chemoactractant for human spermatozoa. However, the presence of beta-chemokine receptors on human spermatozoa has never been reported. By cytometric, Western blotting and immunofluorescence analysis, we demonstrate the presence of CCR5 and CCR3 on ejaculated spermatozoa from healthy subjects. CCR5 was detected in the periacrosomal region of the sperm surface, whereas CCR3 was also present in the postacrosomal cap. Individual variability was observed on CCR5 and CCR3 positive sperm percentages. Presence of Delta32+/-) mutation was demonstrated in two subjects expressing CCR5 in half of the ejaculated spermatozoa. Our findings represent the missing information in favor of the possibility that beta-chemokines and their receptors are involved in sperm chemotaxis. Identification of molecular mechanisms of sperm chemotaxis may allow us to identify predictive parameters of sperm fertilizing ability in hypofertile or infertile subjects. Finally, both CCR5 and CCR3 expressed on the sperm cell surface may be involved in HIV-1 adhesion to spermatozoa, thus allowing these cells to perform as virion cellular carriers during sexual transmission of HIV-1 infection.

Chemokines and their receptors in the pathogenesis of allergic asthma: progress and perspective

PURPOSE OF REVIEW

The importance of chemokines and their receptors to development and maintenance of allergic asthma is reflected in the burgeoning amount of literature currently devoted to this topic. Based on a series of selected references published during the last year, this review now summarizes recent advances and discusses the likely implications of these findings.

RECENT FINDINGS

Of particular interest are reports describing novel interactions between chemokines and both eosinophils and mast cells, including a role for CXCL5 (epithelial cell-derived neutrophil-activating peptide-78) and intracellular CCR3. New insights into TH2-cell dominance are presented in reports dealing with a range of chemokines, including CCL3 (MIP-1alpha), CCL4 (MIP-1beta), CCL5 (RANTES), CXCL9 (Mig), and CXCL10 (IP-10). The increasing importance of structural cell participation is emphasized by reports focusing on the eotaxin family (CCL11, CCL24, and CCL26), as well as CCL17 (TARC), CCL22 (MDC), CXCL9 (Mig), and CX3CL1 (Fractalkine). A developing role for nonreceptor regulatory mechanisms is also emphasized by seminal work relating to metalloproteinases, as well as reports focusing on proteoglycans and beta-Arrestin-2. Finally, significant progress in the field of asthma heritability is featured in reports relating to both known and novel genes, including those encoding CCR5 and DPP-10.

SUMMARY

The critical influence of chemokine biology on the outcome of allergic asthma continues to be highlighted in recent reports describing novel mechanisms by which eosinophils are recruited into the lung and local TH2-cell dominance is maintained. Also of considerable interest is the increasing emphasis currently being realized for structural cell participation, nonreceptor regulatory mechanisms, and the influence of susceptibility genes.

The role of the mast cell in asthma: induction of airway hyperresponsiveness by interaction with smooth muscle?

In a recent study, the difference between asthma and eosinophilic bronchitis (a condition characterized by cough but not airway hyperresponsiveness or airflow obstruction) was infiltration of airway smooth muscle (ASM) by mast cells. Mast cells produce a variety of lipid mediators, chemokines, cytokines, and enzymes that may interact with ASM cells to cause hyperreactivity to constrictive stimuli and proliferation, and activated ASM can produce stem cell factor and other chemokines, cytokines, and growth factors that may act in recruitment, differentiation, and retention of mast cells. Mast cell infiltration of the airways in asthma is T-cell-dependent, and TH2 cytokines from T cells and other sources act in mast cell expansion from circulating and tissue precursors. The recent data on interactions of mast cells and ASM suggest that this could be an important contributor to airway hyperresponsiveness in asthma. Why this occurs in asthma and how it is sustained remain to be established.

Expression of the type 2 receptor for cysteinyl leukotrienes (CysLT2R) by human mast cells: Functional distinction from CysLT1R

Cysteinyl leukotrienes (cysLTs) mediate vascular leakage and bronchoconstriction through the smooth muscle-associated CysLT type 1 receptor (CysLT1R), one of at least two loosely homologous cysLT-binding G protein-coupled receptors. We previously reported that CysLT1R is expressed by cultured human mast cells (hMCs), and that priming these cells with IL-4 enhances their sensitivity to calcium flux and cytokine generation in response to cys-LTs and the nucleotide ligand, uridine diphosphate (UDP), without increasing their surface expression of CysLT1R. We now report that hMCs express the type 2 receptor for cysLTs (CysLT2R) as well, and that the amount of surface CysLT2R protein increases in response to priming with IL-4. The selective function of CysLT2R was evident based on uninhibited IL-8 secretion by IL-4-primed hMCs stimulated with cys-LTs or UDP in the presence of the selective CysLT1R antagonist MK571. MK571 did inhibit IL-5 generation, calcium flux, and phosphorylation of extracellular signal-regulated kinase. IL-8 secretion was inhibited by pertussis toxin and a selective p38 kinase inhibitor, SB203580. The CysLT2 response may permit the cys-LTs and nucleotides generated in infection and tissue injury to elicit IL-8 generation by hMCs, potentially leading to neutrophilic infiltration, a characteristic of aerosol challenge-induced late-phase responses and of sudden death associated with asthma.

Drug development for asthma

Asthma is characterized by abnormal immune cell accumulation and activation in the airways as well as dysfunction of specialized parenchymal cells. Research strategies to define asthma pathogenesis have focused on the hypothesis that this altered state is a consequence of an excessive allergen-driven response. Drug development for asthma has been directed at improving existing agents and expanding new modalities that target the Th2 allergic cascade. Significant opportunities are being pursued in each of these areas. However, this strategy may not account for some critical aspects of asthma pathogenesis. Alternative considerations include the need for a multidisciplinary approach to dissect the complexity of the asthma phenotype as well as a better understanding of nonallergic factors (especially viral reprogramming of airway behavior) in the development of the phenotype. Each of these considerations may provide an alternative strategy for further drug development for asthma and other complex diseases.